On Fri, 30 Nov 2012 15:01:26 +0800 Lin Feng <linfeng@xxxxxxxxxxxxxx> wrote: > > > On 11/30/2012 01:57 PM, Andrew Morton wrote: > > On Fri, 30 Nov 2012 11:42:05 +0800 Lin Feng <linfeng@xxxxxxxxxxxxxx> wrote: > > > >> hi Andrew, > >> > >> On 11/30/2012 07:39 AM, Andrew Morton wrote: > >>> Tricky. > >>> > >>> I expect the same problem would occur with pages which are under > >>> O_DIRECT I/O. Obviously O_DIRECT pages won't be pinned for such long > >>> periods, but the durations could still be lengthy (seconds). > >> the offline retry timeout duration is 2 minutes, so to O_DIRECT pages > >> seem maybe not a problem for the moment. > >>> > >>> Worse is a futex page, which could easily remain pinned indefinitely. > >>> > >>> The best I can think of is to make changes in or around > >>> get_user_pages(), to steal the pages from userspace and replace them > >>> with non-movable ones before pinning them. The performance cost of > >>> something like this would surely be unacceptable for direct-io, but > >>> maybe OK for the aio ring and futexes. > >> thanks for your advice. > >> I want to limit the impact as little as possible, as mentioned above, > >> direct-io seems not a problem, we needn't touch them. Maybe we can > >> just change the use of get_user_pages()(in or around) such as aio > >> ring pages. I will try to find a way to do this. > > > > What about futexes? > hi Andrew, > > Yes, better to find an approach to solve them all. > > But I'm worried about that if we just confine get_user_pages() to use > none-movable pages, it will drain the none-movable pages soon. Because > there are many places using get_user_pages() such as some drivers. Obviously we shouldn't change get_user_pages() for all callers. > IMHO in most cases get_user_pages() callers should release the pages soon, > so pages allocated from movable zone should be OK. But I'm not sure if > we get such rule upon get_user_pages(). > And in other cases we specify get_user_pages() to allocate pages from > none-movable zone. > > So could we add a zone-alloc flags when we call get_user_pages()? Well, that's a fairly low-level implementation detail. A more typical approach would be to add a new get_user_pages_non_movable() or such. That would probably have the same signature as get_user_pages(), with one additional argument. Then get_user_pages() becomes a one-line wrapper which passes in a particular value of that argument. But that means we'd also have to add get_user_pages_fast_non_movable() and things might become a bit stupid. A better approach might be to add a new library function which callers can use before (or after?) calling get_user_pages[_fast](). Unsure. It's the sort of thing where one has to dive in and try a few things. -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
